In various industrial processes that involve fluid material, knowing the properties of the fluids involved is useful. These fluid properties include, for example, speed of sound, density, compressibility, reflectance, acoustic impedance, viscosity, and attenuation. Knowledge of the values of these various properties can be used to adjust process parameters or warn of impending calamity. In many applications, such as oil and gas well (borehole) drilling, fluid density is of particular interest. It is important to know the density of drilling fluid (also referred to as drilling mud) during a drilling operation, in order to prevent a blowout of the well.
In a drilling operation, drilling fluid is pumped down the drill string (essentially a very long pipe), exits at the drill bit, and then returns to the surface within an annulus formed between the outside of the pipe and the inside of the borehole. As the bit drills into the geologic formations, it passes through zones containing various fluids, including lightweight fluids such as saltwater, oil (hydrocarbons), and natural gas. If the pressure within the zone is greater than the pressure within the borehole, these fluids will enter the borehole and mix with the drilling fluid. When lightweight fluids mix with drilling fluid, its density decreases. If the total weight of fluid within the borehole decreases too much, it can lead to a blowout when a high-pressure zone is entered. Accurately monitoring the density of the drilling fluid is therefore very important. In producing wells the fluid density, with other measurements, is used to infer the proportions of oil, water, and natural gas that the well is producing at various depths in the well. Logging tools for measuring fluid density are well known.
An ultrasonic radial scanner tool measures the acoustic impedance of materials immediately behind the casing in a well bore, from which density and other properties may be inferred. Typically, an ultrasonic transducer mounted in a rotating head is used to make the measurement of acoustic impedance behind the casing wall. This measurement is typically made by using an ultrasonic pulse to excite the casing wall in the thickness mode of vibration and measuring the energy content of the returning ultrasonic wave's amplitude. The values for acoustic impedance are then used to identify the material behind the casing. The measurement is to some degree affected by the acoustic impedance of the fluid inside the casing. A more accurate result for the measurement of the acoustic impedance of the material behind the casing would be achieved if it were corrected for the influence of the acoustic impedance of the fluid inside the casing.
U.S. Pat. Nos. 4,685,092 and 6,041,861 describe methods to correlate acoustic impedance of the well bore fluid to the speed of sound in that fluid. U.S. Pat. No. 6,050,141 describes a method for measuring the acoustic impedance of the fluid in a wellbore, particularly of wet cement in wells being prepared for abandonment.
FIG. 1 is a diagram of a fluid transducer portion 100 of a prior art ultrasonic radial scanner tool from Weatherford International, Inc., the assignee of the present application. In this tool, a second transducer 110 is mounted in a fixed location in housing 150 and uses a plate 120 of known properties and distance from the transducer 110 as a reference target. The housing 150 is open to the wellbore fluid, allowing wellbore fluid to enter chambers 130 and 140, so that plate 120 has wellbore fluid on both sides. The transducer 110 and target plate 120 are used to measure the speed of sound of the wellbore fluid inside the well casing. The speed of sound is then used with the time of flight information from the transducer in the rotating head to determine the inside diameter of the casing.
There was a belief in the past that good measurements of wellbore fluid acoustic impedance could be obtained by measuring the decay of the returning ultrasonic waves from the plate 120 with wellbore fluid on both sides, using just the transducer 110 and the known plate 120. However, when this was attempted, inconsistent results were obtained. A better system for performing these measurements would be desirable.